SRSF3-Mediated Ki67 Exon 7-Inclusion Promotes Head and Neck Squamous Cell Carcinoma Progression via Repressing AKR1C2

Surface PD-1 expression in T cells is suppressed by HNRNPK through an exonic splicing silencer on exon 3

OBJECTIVE

Immunotherapy targeting programmed cell death 1 (PDCD1 or PD-1) and its ligands has shown remarkable promise and the regulation mechanism of PD-1 expression has received arising attention in recent years. PDCD1 exon 3 encodes the transmembrane domain and the deletion of exon 3 produces a soluble protein isoform of PD-1 (sPD-1), which can enhance immune response by competing with full-length PD-1 protein (flPD-1 or surface PD-1) on T cell surface. However, the mechanism of PDCD1 exon 3 skipping is unclear.

METHODS

The online SpliceAid program and minigene expression system were used to analyze potential splicing factors involved in the splicing event of PDCD1 exon 3. The potential binding motifs of heterogeneous nuclear ribonucleoprotein K (HNRNPK) on exon 3 predicted by SpliceAid were mutated by site-directed mutagenesis technology, which were further verified by pulldown assay. Antisense oligonucleotides (ASOs) targeting the exonic splicing silencer (ESS) on PDCD1 exon 3 were synthesized and screened to suppress the skipping of exon 3. The alternative splicing of PDCD1 exon 3 was analyzed by semiquantitative reverse transcription PCR. Western blot and flow cytometry were performed to detect the surface PD-1 expression in T cells.

RESULTS

HNRNPK was screened as a key splicing factor that promoted PDCD1 exon 3 skipping, causing a decrease in flPD-1 expression on T cell membrane and an increase in sPD-1 expression. Mechanically, a key ESS has been identified on exon 3 and can be bound by HNRNPK protein to promote exon 3 skipping. Blocking the interaction between ESS and HNRNPK with an ASO significantly reduced exon 3 skipping. Importantly, HNRNPK can promote exon 3 skipping of mouse Pdcd1 gene as well.

CONCLUSIONS

Our study revealed a novel evolutionarily conserved regulatory mechanism of PD-1 expression. The splicing factor HNRNPK markedly promoted PDCD1 exon 3 skipping by binding to the ESS on PDCD1 exon 3, resulting in decreased expression of flPD-1 and increased expression of sPD-1 in T cells.

The Prognostic Importance of Ki-67 in Gastrointestinal Carcinomas: A Meta-analysis and Multi-omics Approach

PURPOSE

This study aimed to determine if Ki-67, a commonly used marker to measure tumor proliferation, is a reliable prognostic factor in various types of gastrointestinal (GI) cancers based on current high-quality multivariable evidence.

METHODS

A comprehensive search was conducted in PubMed, Embase, Scopus, and ISI Web of Science databases to investigate the association between Ki-67 positivity and overall survival (OS) and disease/recurrence-free survival (DFS/RFS) in GI cancers. Heterogeneity was assessed using Chi-square-based Q and I2 analyses and publication bias using funnel plots and Egger's analysis. In addition, Ki-67 levels in different GI cancers were examined by different platforms. The prognostic capability of Ki-67, gene ontology (GO), and pathway enrichment analysis were obtained from GEPIA2 and STRING.

RESULTS

Totally, 61 studies, involving 13,034 patients, were deemed eligible for our evaluation. The combined hazard ratios (HRs) demonstrated the prediction ability of overexpressed Ki-67 for a worse OS (HR: 1.67, P < 0.001; HR: 1.37, P = 0.021) and DFS/RFS (HR: 2.06, P < 0.001) in hepatocellular and pancreatic malignancies, respectively, as confirmed by multi-omics databases. However, similar correlation was not found in esophageal, gastric, and colorectal cancers. Furthermore, most of the associations were identified to be robust based on different subcategories and publication bias assessment. Finally, enriched Ki-67-related genes were found to be involved in various important signaling pathways, such as cell cycle, P53 signaling network, and DNA damage responses.

CONCLUSION

This study supports that Ki-67 can serve as an independent prognostic biomarker for pancreatic and hepatocellular malignancies in clinical settings.

An anti-PD-1 antisense oligonucleotide promotes the expression of soluble PD-1 by blocking the interaction between SRSF3 and an exonic splicing enhancer of PD-1 exon 3

PD-1 is a key immune checkpoint molecule. Anti-PD-1 immunotherapy is encouraging in cancer treatment. However, it still needs to be improved. PD-1 has at least five isoforms generated by alternative splicing. An isoform without exon 3 encoding soluble PD-1 (sPD-1) can activate anti-tumor immunity by inhibiting the interaction between cellular surface full-length PD-1 (flPD-1) and PD-L1. However, the regulatory mechanism of exon 3 splicing remains largely unknown. Here, we screened the exon 3 sequence by mutation and searched corresponding splicing factors by SpliceAid database and pulldown assay. The alternative splicing of PD-1 exon 3 was analyzed by RT-PCR. The expression levels of flPD-1 and sPD-1 were analyzed by Western blot, flow cytometry, and ELISA. We discovered that an exonic splicing enhancer (ESE) of exon 3 is essential for its inclusion. Moreover, SRSF3 can bind to this ESE and enhance exon 3 inclusion and flPD-1 expression. We designed and screened out an antisense oligonucleotide (ASO) targeting PD-1 to block the interaction between SRSF3 and ESE, and significantly increase exon 3 skipping and sPD-1 expression, which was verified in various tumor cells in addition to oral cancer cells. Altogether, our results uncovered the regulatory mechanism of human PD-1 exon 3 splicing and sPD-1 expression and further designed a novel anti-PD-1 ASO, which are useful for developing a new method of anti-cancer immunotherapy.